Many people erroneously assume that if the child is hyperactive, the brain must also be overactive. This is not the case, however.
In the picture above, high levels of activity are indicated by red, orange, and yellow, while lower levels of activity are indicated by greens and blues. When given a problem to think about, the "normal" control shows a pattern of activity in the brain, but the brain of the individual with ADHD is not as active.
It is the underactivity in the "braking" systems in the brain that leads to overactive behavior, impulsivity, and/or inability to sustain attention. Normally, we would stop ourselves from being distracted, but the "brakes are off" in the ADHD situation.
Understanding that the problem is underactivity in certain neural circuits (as well as possibly smaller brain volumes in certain structures in the brain) helps educators understand what are often called the paradoxical effects of stimulant medications such as Ritalin. Some people ask, "Isn't giving a hyperactive child a stimulant like bringing coals to Newcastle?" The answer is "No! Giving a hyperactive child a stimulant will boost their brain activity up into the 'normal' region, so they will be able to stop themselves from responding to distractions, behaving impulsively, or just being hyperactive."
The brain scan shown on this page is from the research of Dr. Zametkin (1990).